This is the first sail ever deployed in space, and if they succeed in using it for solar-sail flight — it’ll still be a few weeks before we know that — it’ll be a milestone,” said Louis Friedman, executive director of the Planetary Society, an organization dedicated to promoting space exploration, which is readying its own solar-sailing mission.

A solar sail uses the pressure from photons striking its surface to push the spacecraft through space. Materially, the 650 square-foot sail is made of incredibly thin, aluminized plastic that’s only 0.0003 inches thick, a little thicker than spider silk, or about the diameter of a red blood cell. When a photon strikes its surface, it bounces off, imparting its momentum to the sail. Each photon might not deliver much thrust, but over time, all that light adds.

In the IKAROS design, the sail was unfurled by using centrifugal force generated by spinning the craft.

Space-travel proponents are particularly interested in the technology because it doesn’t require fuel, which makes it the leading candidate for long distance space travel.

Earlier in the deployment, the sail is still folded.

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